add JAVA_HOME check for posix window launcher

[agwhitaker93]

Adds a check to see if the file $JAVA_HOME/bin/java exists in the posix HDOS and Runelite launcher functions, using std::filesystem::exists

The error message could probably be better, but this is what it currently looks like:

[Adamcake]

In the second commit, 100 chars isn't enough to hold the message you're printing into it, so you're writing to undefined memory. Consider PATH_MAX as a buffer size and using snprintf instead of sprintf to limit the write length.